Electronic circuits

ABSTRACT

A high voltage AC power supply circuit for a capacitive load C L , such as an electroluminescent lamp, includes a low voltage DC supply, an inductor L and a FET S in series. The FET S can be pulsed so that the inductor L generates a voltage to charge the capacitive load C L  via an H-bridge H, which is in parallel with the FET S. A diode D prevents current discharging from the capacitive load C L  while the FET S is closed. The total capacitance downstream of the diode D and in parallel with the capacitive load C L  is less than the capacitive load C L , so that when the polarity of the H-bridge is reversed, the voltage across the H-bridge collapses to earth and the capacitive load C L  is discharged via the low voltage DC supply. The circuits which a employ a large smoothing capacitor in parallel with the H-bridge.

TECHNICAL FIELD

The present invention relates to electronic circuits, and in particularto electronic circuits which can be used in a high voltage power supplyfor a capacitive load, such as an electroluminescent lamp.

BACKGROUND OF THE INVENTION

Electroluminescent lamps generally comprise a layer of phosphormaterial, such as a doped zinc sulphide powder, between two electrodes.It is usual for at least one electrode to be composed of a transparentmaterial, such as indium tin oxide (ITO), provided on a transparentsubstrate, such as a polyester or polyethylene terephthalate (PET) film.The lamp may be formed by depositing electrode layers and phosphorlayers onto the substrate, for example by screen printing, in which caseopaque electrodes may be formed from conductive, for examplesilver-loaded, inks. Examples of electroluminescent devices aredescribed in WO 00/72638 and WO 99/55121.

An electroluminescent lamp of the general type described above isilluminated by applying an alternating voltage of an appropriatefrequency between the electrodes of the lamp to excite the phosphor.Commonly, the phosphors used in electroluminescent lamps require avoltage of a few hundred volts. Typically, such electroluminescent lampsmay have a capacitance in the range 100 pF to 1 μF.

The inventors have been involved in the development ofelectroluminescent displays which comprise electroluminescent lampshaving selectively illuminable regions for displaying information. Suchdisplays have the advantage that they can be large, flexible andrelatively inexpensive. In the context of such electroluminescentdisplays, the inventors have sought to provide a simple power supplyarrangement for an electroluminescent lamp or display.

A known type of circuit for producing a higher output voltage from a lowvoltage DC supply is a “flyback converter”. Such a circuit comprises aninductor and an oscillating switch arranged in series. In parallel withthe oscillating switch, a diode and a capacitor are arranged in series.The switch oscillates between an open state and a closed state. In theclosed state, a current flows from the DC supply through the inductorand the switch. When the switch is opened, the current path isinterrupted, but the magnetic field associated with the inductor forcesthe current to keep flowing. The inductor therefore forces the currentto flow through the diode to charge the capacitor. The diode preventsthe capacitor discharging while the switch is closed. The capacitor cantherefore be charged to a voltage which is higher than the DC supplyvoltage, and current at this voltage can be drawn from the capacitor.

In order to supply an alternating current to a load from a flybackconverter, an H-bridge may be provided in parallel with the capacitor.In general, an H-bridge comprises two parallel limbs, each limb having afirst switch in series with a second switch. On each limb between thefirst and second switches there is a node, and the load is connectedbetween the respective nodes of the limbs. Current can flow through theload in one direction via the first switch of one limb and the secondswitch of the other limb and in the other direction via the other twoswitches. The switches of the H-bridge are operated so that currentflows through the load first in one direction and then in the other.

When an H-bridge is used to supply a capacitive load C_(L) with a supplyvoltage V, during the first half of the cycle of operation, the loadC_(L) is at +V. When the H-bridge switches and reverses the polarity ofthe load, there is a potential difference of −2V between the supplyvoltage and the load. The load is supplied rapidly with current from thesupply until there is no potential difference, and this requires2C_(L)V² of energy. Similarly, when the H-bridge is switched to returnthe load to the original polarity at the end of the cycle, a further2C_(L)V² of energy is required to bring the load back to +V.

It will be seen, therefore, that each cycle of the operation of theH-bridge requires 4C_(L)V² of energy. The power consumption, assuming100% efficiency, is 4C_(L)V²f, where f is the cycling frequency of theH-bridge. This represents a significant power consumption when thefrequency and the voltage are large.

It is usual to provide a large smoothing capacitor (such as thecapacitor of the flyback converter described above) in parallel with theH-bridge in order to provide current for the rapid charging anddischarging of the capacitive load. The smoothing capacitor protects thepower supply from the large currents which result from the switching ofthe polarity of the H-bridge, and ensures that the supply voltage doesnot drop significantly.

The inventors have realised, however, that if the smoothing capacitor ischosen to be smaller than the capacitive load, when the polarity of theH-bridge is switched, the current drawn from the smoothing capacitor bythe capacitive load fully discharges the smoothing capacitor and thehigh voltage supply collapses. In this case, almost immediately afterthe H-bridge has been switched, the current supplied to the capacitiveload is drawn directly from the low voltage DC supply, rather than froma store of charge at high voltage in a large smoothing capacitor.

SUMMARY OF THE INVENTION

The present invention provides an electronic circuit for supplying acapacitive load with an alternating high voltage from a lower voltage DCsupply, the circuit comprising:

an H-bridge having two parallel limbs, each limb having a firstswitching element in series with a second switching element and a nodebetween the first and second switching elements, the capacitive loadbeing connected, in use, between the respective nodes of the limbs;

a converter powered by the low voltage DC supply and arranged to supplycurrent to the H-bridge to charge the capacitive load to a voltage whichis higher than the DC supply voltage; and

a diode arranged in series between the converter and the H-bridge toprevent current flowing back from the charged capacitive load, wherein

the switching elements of the H-bridge are controlled alternately suchthat in a first condition the first switching elements of one limb andthe second switching elements of the other limb conduct to supplycurrent from the converter to the capacitive load in one direction, andin a second condition the other two switching elements of the limbsconduct to supply current from the converter to the capacitive load inthe opposite direction, and

the total capacitance provided in parallel with the H-bridge downstreamof the diode is less than the capacitance of the capacitive load.

In accordance with this arrangement, when the H-bridge is switched fromthe first condition to the second condition or vice versa, at least someof the current which recharges the reversed capacitive load is drawnfrom the circuit upstream of the diode, where the voltage is lower. Inthis way, the power consumption required to operate the circuit issignificantly reduced compared to an arrangement in which current issupplied to recharge the capacitive load from a large smoothingcapacitor.

Preferably, the total capacitance provided in parallel with the H-bridgedownstream of the diode is less than 50% of the capacitance of thecapacitive load. In a preferred arrangement, this capacitance is between10% and 20% of the capacitance of the capacitive load.

A smoothing capacitor may be provided upstream of the diode in parallelwith the H-bridge in order to compensate for the imperfect switching ofthe switching elements of the H-bridge. However, the capacitance of theswitching capacitor is kept small in accordance with the invention.

The diode may be any suitable device which allows current flow in onedirection only over the range of operating voltages of the circuit andthe term “diode” is used herein accordingly. The role of the diode is toallow a higher voltage than the DC supply voltage to be stored on thecapacitive load without current flowing back from the capacitive loadtowards the converter resulting in discharge of the capacitive load.

The switching elements may be any suitable switching devices and, ingeneral, are transistors. In the preferred arrangement, the switchingelements are field effect transistors (PETs). In a particularlypreferred arrangement, the first switching elements are p-channel FETsand the second switching elements are n-channel FETs.

The operation of the switching elements of the H-bridge may becontrolled by any suitable means. In a preferred arrangement, a polarityvoltage is applied to the switching elements, for example to the gatesof the FETs. The polarity voltage may be a pulse width modulated signal.Thus, the circuit may further comprise an oscillator arranged togenerate the polarity voltage. In a particularly convenient arrangement,the signal from the oscillator may also be used by the converter inorder to provide synchronised operation of the converter and theH-bridge, optionally by means of a divider. Typically, the frequency ofthe polarity voltage is in the range 50 Hz to 10 kHz.

The converter may be any suitable converter such as a forward converteror a flyback converter. In a preferred arrangement, the converter is aflyback converter.

The flyback converter may comprise an inductive element and an outputswitching element arranged in series. The output switching element isarranged to alternate, in use, between a first state and a second state,whereby in the first state a current path is provided through theinductive element and the output switching element, which current pathis interrupted in the second state, such that when the output switchingelement changes from the first state to the second state, the inductiveelement generates a voltage at an output of the circuit for charging acapacitive load. An output diode may prevent current flowing back fromthe output while the output switching element is in the first state.

The inductive element may be any suitable component which is capable ofoperating in the required manner, such as an inductor or coil.Typically, the inductive element may have an inductance in the range 50μH to 50 mH, for example 470 μH.

The output diode may be any suitable device which allows current flow inone direction only over the range of operating voltages of the circuit.The role of the output diode is to allow a higher voltage than the DCsupply voltage to be stored on the capacitive load without currentflowing back from the capacitive load towards the inductive element.

The output switching element may be any suitable switching device and,in general, is a transistor. In the preferred arrangement, the switchingelement is a field effect transistor (FET). In a particularly preferredarrangement, the output switching element is an n-channel FET.

Advantageously, the output switching element may be connected directlyto earth potential. According to this arrangement, the switching elementis not required to be able to switch at high voltage, which simplifiesthe design of the circuit.

The operation of the output switching element may be controlled by anysuitable means. In a preferred arrangement, a control voltage is appliedto the switching element, for example to the gate of the FET. Thecontrol voltage may be a pulse width modulated signal. Typically, thefrequency of the control voltage is in the range of 10 to 100 kHz. Thecircuit may further comprise an oscillator arranged to generate thecontrol voltage. In a particularly convenient arrangement, the signalfrom the oscillator may also be used by the H-bridge in order to providesynchronised operation of the circuit and the H-bridge.

Current may be supplied to the converter from a DC supply. Thus, thecapacitive load may be charged from the DC supply by means of theinductive element and the output switching element.

Typically, the DC supply has a voltage of less than 100 V, for examplein the range 2 to 24 V. The capacitive load may be charged to a peakvoltage between 5 to 500 times that of the supply voltage. Typically,the peak voltage is in the range 10 to 100 times that of the supplyvoltage.

The output switching element may be arranged to alternate between thefirst and the second state at a frequency which is a multiple of thefrequency at which the H-bridge alternates between the first conditionand the second condition. In this way, the switching signal to theswitching elements of the converter and the H-bridge can be generatedfrom the same oscillator, for example using a divider.

In the preferred arrangement, the capacitive load is anelectroluminescent lamp.

These and other features of the present invention will become apparentupon review of the following detailed description of the invention whentaken in conjunction with the drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention will now be described by way ofexample only and with reference to the accompanying drawings, in which:

FIG. 1 a and FIG. 1 b represent the operation of an H-bridge inaccordance with the invention;

FIG. 2 a and FIG. 2 b illustrate the operation of a flyback converterfor use with the invention; and

FIG. 3 illustrates the operation of a preferred embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the embodiments described, corresponding reference signs have beenused to indicate corresponding components.

Referring to FIG. 1 a, there is shown an electronic circuit inaccordance with the invention. The circuit comprises a current source Iin series with a diode D and an H-bridge arrangement H. A smoothingcapacitor C_(S) is provided in parallel with the H-bridge arrangement Hand is connected to earth potential.

The H-bridge arrangement H in FIG. 1 a comprises four switching elementsS_(A) to S_(D) which are represented as simple switches for reasons ofclarity. In a practical circuit, the switches S_(A) to S_(D) areprovided by field effect transistors (FETs). The H-bridge comprises twoparallel limbs each having two switches S_(A), S_(D) and S_(C), S_(S)arranged in series. A capacitive load C_(L) in the form of anelectroluminescent lamp is connected between the limbs of the H-bridgeat nodes on each limb which are between the switches of the limb. TheH-bridge is connected to earth potential at one end.

The positions of the switches S_(A) to S_(D) are controlled by apolarity voltage V_(P), the variation of which over time is representedin FIGS. 1 a and 1 b. When V_(P) is low, switches S_(A) and S_(D) areopen and do not conduct and switches S_(C) and S_(D) are closed andconduct. This situation is shown in FIG. 1 a. When V_(P) is high,switches S_(A) and S_(B) are closed and conduct while switches S_(C) andS_(D) are open and do not conduct. This situation is illustrated in FIG.1 b.

The operation of the circuit shown in FIGS. 1 a and 1 b will now bedescribed. A converter such as a flyback converter or forward converter,represented as a current source I, supplies current via the diode D tothe smoothing capacitor C_(S) and the capacitive load C_(L). Thedirection in which the capacitive load C_(L) is charged is determined bythe position of the switches S_(A) to S_(D). The capacitors C_(S) andC_(L) continue to be charged until the current source I ceases to supplycurrent. The voltage on the capacitors C_(S) and C_(L) consequentlyrises. Reverse current flow from the capacitors is prevented by thediode D.

Thus, when the capacitive load C_(L) is fully charged to the loadvoltage V_(L), the charge thereon is C_(L)V_(L) and the charge on thesmoothing capacitor is C_(S)V_(L). When the polarity voltage V_(P) goeshigh, as shown in FIG. 1 b, the polarity of the charged capacitive loadC_(L) with respect to the smoothing capacitor C_(S) and the currentsource is reversed. Thus, point Y in FIG. 1 b is at a potential −V_(L)relative to earth potential, while the potential at point X is +V_(L)relative to earth potential. This potential difference causes current toflow until points X and Y are at the same potential.

If the capacitance of the smoothing capacitor C_(S) is large, itsupplies sufficient charge to the capacitive load C_(L) to bring thevoltage on the capacitive load C_(L) up to approximately the loadvoltage V_(L). However, in doing so, the smoothing capacitor C_(S) hasprovided 2C_(L)V_(L) ² of energy to the capacitive load which must bereplaced from the current source I. Thus, for each cycle of theH-bridge, 4C_(L)V_(L) ² of energy is drawn from the current source I.

In accordance with the invention, however, the capacitance of thesmoothing capacitor C_(S) is chosen to be smaller than that of thecapacitive load C_(L), such that when the polarity voltage V_(P) goeshigh, the smoothing capacitor C_(S) does not have sufficient storedcharge to maintain the voltage at point X at the load voltage V_(L) andconsequently the voltage at X and Y collapses to earth potential. Thecapacitive load C_(L) is then charged back to the load voltage V_(L)from the current source I. In this way, current is drawn from thecurrent source I at a relatively low voltage rather than from the largesmoothing capacitor at a high voltage. With a small smoothing capacitorC_(S) only C_(L)V_(L) ² of energy is required per cycle.

FIGS. 2 a and 2 b show an arrangement of a flyback converter forcharging a capacitive load to a high voltage. The flyback convertershown in FIG. 2 can be used with the H-bridge arrangement H shown inFIG. 1, by replacing the components C_(L) downstream of the diode D inFIG. 2 with the components C_(S) C_(L) S_(A)–S_(D) downstream of thediode D in FIG. 1. For the sake of simplicity the capacitive load C_(L)is shown in FIG. 2 without the H-bridge.

As shown in FIG. 2 a, the flyback converter comprises a DC supply inseries with an inductor L and a switch S. The switch S is connectedbetween the inductor and earth potential. In a practical arrangement,the switch S is provided by a field effect transistor, the output FET.However, for the sake of clarity, in FIGS. 2 a and 2 b the switch S isshown as a simple switch.

In parallel with the switch S is provided a diode D in series with thecapacitive load C_(L). The capacitive load C_(L) is arranged between thediode and earth potential.

The switch S is controlled by a switch voltage V_(S) which varies overtime as indicated in FIG. 2 a. When the switch voltage V_(S) is high,the switch S is closed and conducts. This situation is shown in FIG. 2a. When the switch voltage V_(S) is low, the switch S is open and doesnot conduct. This situation is shown in FIG. 2 b.

The circuit shown in FIGS. 2 a and 2 b operates as follows. While theswitch voltage V_(a) is high, as shown in FIG. 2 a, current I flows fromthe DC supply through the inductor L and the closed switch S to earth.Assuming the voltage on the capacitive load C_(L) is higher than the DCsupply voltage, no current flows through the diode D.

When the switch voltage V_(S) goes low, as shown in FIG. 2 b, thecurrent path through the inductor L and switch S is interrupted by theopen switch S. However, the energy stored in the magnetic fieldassociated with the inductor L forces the current I to continue flowingand the inductor L generates a sufficiently high voltage that thecurrent I flows through the diode D to charge the capacitive load C_(L).In this way, with each transition of the switch voltage V_(S) from highto low, the voltage V_(L) on the capacitive load C_(L) is increased, asindicated in FIG. 2 b. The diode D prevents current flow back from thecapacitive load C_(L) to earth or to the DC supply when the switch S isclosed.

It will be seen therefore that the capacitive load C_(L) can be chargedto any desired voltage by applying an alternating switch voltage V_(S)to the switch S.

FIG. 6 shows a circuit in accordance with a preferred embodiment of theinvention. The circuit combines the features of the converter of FIG. 2and the H-bridge of FIG. 1.

The circuit shown in FIG. 3 comprises an inductor L in series with ann-channel FET. The n-channel FET provides the output switch S. The gateof the n-channel FET S is supplied with a control voltage signal V_(C).

The DC supply is arranged in series with the inductor L for supplying acurrent I_(S).

The circuit shown in FIG. 6 further comprises an H-bridge H. A smoothingcapacitor C_(S) is provided in parallel with the H-bridge H and has acapacitance of around 1 nF.

The H-bridge H comprises two parallel limbs. The first limb comprises ap-channel PET S_(A) in series with an n-channel FET S_(D). Between thetwo FETs S_(A) and S_(D) there is a connection for the capacitive loadC_(L), which is an electroluminescent lamp with a capacitance of around10 nF. The gates of the PETs S_(A) and S_(D) are supplied with apolarity voltage V_(P). The other limb of the H-bridge comprises ap-channel FET S_(C) in series with an n-channel FET S_(B). Thecapacitive load C_(L) is connected to a point between the two FETs S_(C)and S_(B). The gates of the FETs S_(C) and S_(B) are supplied with theinverse of the polarity voltage V_(P) by means of an inverter INV.

As indicated by the voltage graphs in FIG. 3, one cycle of the circuitcomprises two distinct, recurring phases. In the first phase, thepolarity voltage V_(P) is high, such that FETs S_(C) and S_(D) conductwhile FETs S_(A) and S_(B) do not conduct. The control voltage V_(C) tothe output FET S pulses so that the output FET S alternately conductsand does not conduct. Consequently, the changing current through theinductor L charges the smoothing capacitor C_(S) and the capacitive loadC_(L), via the FET S_(C). The voltage V_(L) across the capacitive loadC_(L) in the direction of the arrow in FIG. 3 rises due to the increasedcharge on the capacitive load C_(L), as does the voltage V_(HV) at pointX.

In the second phase, the polarity voltage V_(P) goes low, such that theFETs S_(C) and S_(D) cease to conduct and the FETs S_(A) and S_(S) beginto conduct. The polarity of the capacitive load C_(L) relative to thepoint X is therefore reversed. When this change of polarity occurs,current is drawn from the smoothing capacitor C_(S) and subsequently theDC supply to discharge the negative charge on the capacitive load C_(L).

During this phase, the control voltage V_(C) to the output FET S ispulsed so that current is drawn intermittently from the DC supplythrough the inductor L to charge the capacitive load C_(L). However,because the FETs S_(A) and S_(B) are conducting rather than the FETsS_(C) and S_(D), the capacitive load C_(L) is charged with current inthe opposite direction to that in the first phase, so that a negativevoltage relative to the voltage V_(HV) at point X is provided on thecapacitive load C_(L).

Between the second phase and the repeat of the first phase, the polarityvoltage V_(P) goes high. Again, the voltage V_(HV) at point X collapsesand the capacitive load C_(L) is discharged by drawing current from theDC supply.

Thus, it will be seen that according to this arrangement there isprovided a simple, energy efficient power supply for anelectroluminescent lamp.

In summary, a high voltage AC power supply circuit for a capacitive loadC_(L), such as an electroluminescent lamp, includes a low voltage DCsupply, an inductor L and a FET S in series. The FET S can be pulsed sothat the inductor L generates a voltage to charge the capacitive loadC_(L) via an H-bridge H, which is in parallel with the FET S. A diode Dprevents current discharging from the capacitive load C_(L) while theFET S is closed. The total capacitance downstream of the diode D and inparallel with the capacitive load C_(L) is less than the capacitive loadC_(L), so that when the polarity of the H-bridge is reversed, thevoltage across the H-bridge collapses to earth and the capacitive loadC_(L) is discharged via the low voltage DC supply. The circuit has alower power consumption than circuits which a employ a large smoothingcapacitor in parallel with the H-bridge.

It should be apparent that the foregoing relates only to the preferredembodiments of the present invention and that numerous changes andmodification may be made herein without departing from the spirit andscope of the invention as defined by the following claims and theequivalents thereof.

1. An electronic circuit for supplying a capacitive load, such as anelectroluminescent lamp, with an alternating high voltage from a lowervoltage DC supply, the circuit comprising: an H-bridge having twoparallel limbs, each limb having a first switch element in series with asecond switching element and a node between the first and secondswitching elements, the capacitive load being connected, in use, betweenthe respective nodes of the limbs; a converter powered by the lowvoltage DC supply and arranged to supply current to the H-bridge tocharge the capacitive load to a voltage which is higher than the DCsupply voltage; and a diode arranged in series between the converter andthe H-bridge to prevent current flowing back from the charged capacitiveload; and a capacitor provided downstream from the diode in parallelwith the H-bridge, wherein the switching elements of the H-bridge arecontrolled alternately such that in a first condition the firstswitching elements of one limb and the second switching elements of theother limb conduct to supply current from the converter to thecapacitive load in one direction, and in a second condition the othertwo switching elements of the limbs conduct to supply current from theconverter to the capacitive load in the opposite direction, and thetotal capacitance provided in parallel with the H-bridge downstream ofthe diode is less than 50% of the capacitance of the capacitive load. 2.An electronic circuit as claimed in claim 1, wherein the totalcapacitance provided in parallel with the H-bridge downstream of thediode is at least 10% of the capacitance of the capacitive load.
 3. Anelectronic circuit as claimed in claim 1, wherein the total capacitanceprovided in parallel with the H-bridge downstream of the diode isbetween 10% and 20% of the capacitance of the capacitive load.
 4. Anelectronic circuit as claimed in claim 1, wherein the converter comprisean inductive element and an output switching element arranged in series;and the output switching element is arranged to alternate, in use,between a first state and a second state, whereby in the first state acurrent path is provided through the inductive element and the outputswitching element, which current path is interrupted in the secondstate, such that when the output switching element changes from thefirst state to the second state, the inductive element generates avoltage at an output of the circuit for charging a capacitive load.